1. Field of the Invention
The present invention relates to a measuring method of an optical system of an exposure apparatus for use in a lithography process, in semiconductor device manufacturing, particularly to an aberration measuring method of a projection optical system.
2. Description of the Related Art
There have been proposed various methods of simply measuring aberration of a projection optical system of a reduced projection type exposure apparatus for use in manufacturing a semiconductor device without disassembling the apparatus. The aberration can roughly be classified into two types. One is aberration which is referred to as odd function aberration and which causes position deviation of an image. Coma aberration is typical odd function aberration. The other is even function aberration. The even function aberration is aberration which causes a deviation of focus distance of the image. A spherical aberration is typical even function aberration.
In a typical method of using a silicon wafer coated with photoresist and a photomask to simply measure the odd function aberration without disassembling the exposure apparatus being operated, a method is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 11-237310. This method generally has an advantage that an overlay inspection apparatus or optical microscope for use in an inspection process of semiconductor device manufacturing can be used to simply obtain aberration amount. However, a limitation of a measurable region described hereinafter is clarified. That is, when a diffraction angle is reduced and a 2nd-order or higher-order diffracted light contributes to image forming (when a three-beam interference condition is not established), the aberration cannot be measured. In other words, there has been a problem that wavefront aberration amount cannot be measured in a region in the vicinity of a middle of a projection pupil of a projection optical system.